Metal oxide semiconductor (MOS) devices have been applied extensively in various power switch components, and the high breakdown voltage and the low on-resistance are two main features required for these applications.
For the general power MOS device, the on-resistance of the MOS device depends on the thickness of the epitaxial layer, whereas, the breakdown voltage also depends on the thickness of the epitaxial layer. The way of increasing the breakdown voltage by increasing the thickness of the epitaxial layer may result in the increasing of the on-resistance. On the other hand, the way of reducing the on-resistance by decreasing the thickness of the epitaxial layer may lower down the breakdown voltage. Thus, the high breakdown voltage and the low on-resistance cannot be taken care concurrently.
To improve the shortcomings of the general MOS device and find out a solution of accessing the low on-resistance and the high breakdown voltage simultaneously, new-generation MOS devices with a super junction structure has been disclosed. For example, the article “COOLMOS—a new milestone in high voltage power MOS” published in the Power Semiconductor Devices and ICs by L. Lorenz, G. Deboy in 1999 teaches a MOS device with the super junction structure being named as COOLMOS, which is a registered trademark of Infineon Technologies AG.
With reference to FIG. 1 for a MOS device having a super junction structure, the MOS device includes an N-type cylindrical structure 13 and a P-type cylindrical structure 15 alternately formed on an N-type base 11, wherein the N-type cylindrical structure 13 constitutes a current path. As the drain and the source of the MOS device are reverse biased, a depletion region between the N-type cylindrical structure 13 and the P-type cylindrical structure 15 is expanded transversely to cut off the electrically conduction path. As mentioned, the on-resistance of the MOS device depends on the doping concentration of the N-type cylindrical structure 13, and the breakdown voltage depends on the thickness of the epitaxial layer. The increasing of the doping concentration of the N-type cylindrical structure 13 for reducing the on-resistance would not lower down the breakdown voltage. Accordingly, the MOS device is capable to prevent the breakdown voltage from being reduced when lowering the on-resistance, and thus can achieve the effect of suppressing the on-resistance and provide a voltage withstanding property higher than that of the traditional power MOS device.
However, the super junction structure as shown in FIG. 1 requires a complicated fabrication process, which includes a plurality of epitaxial layer growing steps for growing the epitaxial layers A˜F, and the respective lithographic and patterning steps, and the respective ion-implantation steps. In addition, each lithographic step requires an aligning procedure to have the doping regions formed in the epitaxial layers A˜F aligned with each other to complete the cylindrical structure, and thus the prior art incurs high production cost and long manufacturing time.
Therefore, it is a main subject for the present invention to use a simplified manufacturing process to produce a MOS device with a high breakdown voltage and a low on-resistance.